The present invention relates to a fluid filtering device and particularly to a filter for separating liquid from gases. More specifically, the present invention is concerned with a filtering device utilizable with a gas analyzer or monitor, having a very limited effect on the response time of the monitor to changes of the gas content and being independent of the orientation of the device or of the field of gravitation acting thereupon.
The current state of the art of filters for gas analyzers can be divided into two categories:--technologies based on hydrophobic microporous membranes, and mechanical separators.
Gas filters based on the first technology commonly use flat sheets of hydrophobic microporous membranes embodied within an appropriate housing, whose structure is designed to both collect the trapped liquid and for connection with the analyzers or monitors.
In many applications (specifically capnographs--CO.sub.2 respiration monitors), it is required that the gas flows through the filtering media without the wave form of the gas being disturbed. This is required so that the measured changes in time of the gas constitution originate only from the breath under test and not those introduced later by parts of the flow system, which transfers the gas to the measuring region. A measure of merit for a filtering device reflecting its ability to freely transfer gases is defined by the response time it invokes. In general, the response time is a measure of response to a change in gas constituent. A long response time will distort the output display even at low breathing rates and such distortion may affect the accuracy of the tested information regarding a patient's health condition displayed on, or recorded by, the analyzer or monitor.
When working with low gas flow rates, as is typical with capnographs, in order to create a gas filter with minimum distortion to the gas wave form providing short response times, the filter should provide minimal turbulence and resistance to a laminar flow and have no fluid passageways through which the gas passes, which due to shape, size or material thereof, disturb the wave form. For example, the shape and size of the fluid passageways must be such that no abrupt changes in dimensions or volume thereof when entering the filter be permitted. Otherwise, the gas entering enlargements in the passageways will possess very different flow rates along the radius perpendicular to the direction of gas flow upstream therefrom. Regions furthest away from the center of the passageways may have very slow flow rates relative to the center. Such a distribution of fluid flow rates will mix gases coming at different times from the patient and hence impair the response time. To an extent, this disturbance of smooth flow is dependent on the size and abruptness of the enlargement. When using flat sheet membranes for filtering purposes, in order to provide as large a surface area of the membrane material as required, a large volume with abrupt changes in dimensions is inevitable.
Further, if the gas must pass either between or through porous walls (used to either soak up liquid or as a filter media) which have a substantial thickness (e.g. more than 0.5 mm), then the free and unimpaired flow of gas will be disturbed by parts of the gases diffusing within and outside of the porous walls, where the flow is hindered relative to the flow of the gases not entering the porous material, thereby causing a mixing of the gas.
Hence, distortion to smooth unimpaired gas flow in the prior art fluid filters are caused by three major factors:
a) by the materials of the filtering device itself, including the porous material of the filtering membrane and porous portions of the walls thereof, to an extent proportional to their thickness; PA1 b) by the shape or configuration of the filter body itself, presenting abrupt changes in the gas passageway between inlet and outlet thereof, and PA1 c) by the overall size of the volume or space of the passageway for the gas flow from the inlet to the outlet of the filter.
These three factors relating to the passageway where the fluid to be filtered inside the filter traverses, is also referred to herein as "space". It has been found that in order to achieve effective analysis of the characterisitics of the filtered fluid, the filter should have as little "dead space" as possible, namely, the space in which the above three factors prevail and which factors are detrimental to the optimal analysis, should be minimal.
In attempting to overcome the disadvantages of the above-described fluid filters, in certain instances there has been applied means for continuously removing the trapped liquid away from the membrane surface to be collected/discarded by a separate feature of the monitor. This more complicated solution, which permits the use of a smaller sized membrane, still requires a minimum sized flat membrane in order not to produce too high a resistance to flow with its inevitable drawbacks of undesired sizes and shapes and obligates a defined orientation for its correct operation. This pre-defined orientation, which is also a fundamental requirement in mechanical filters, is not always possible for implementation since, in many applications, for example, transportable monitors, the filter must operate while the monitor is oriented in any direction.
The filtering device of the present invention is especially advantageous for filtering small or minute volumes of fluid, for example, fluids extracted for analysis from neonatals and the aged.
It is therefore a broad object of the present invention to provide a fluid filtering device to be used in conjunction with a gas monitor or analyzer having a construction having a limited interference to the gas flow.
It is a further broad object of the present invention to provide a fluid filtering device to be used in conjunction with a gas monitor or analyzer, whose orientation will not impede the proper operation of the filtering device, and which is independent of the field of gravity acting thereupon.
It is a still further object of the present invention to provide a fluid filtering device having means for trapping and collecting filtered out non-gaseous components, without the trapped and collected components impairing the gas flow.